doc: various update to command documents

1. consistent usage section (go tool xxx)
        2. reformat cmd/ld document with minor correction
           document which -H flags are valid on which ld
           document -d flag can't be used on Windows.
           document -Hwindowsgui

R=golang-dev, r, rsc
CC=golang-dev
https://golang.org/cl/5782043

src/cmd/gc/doc.go

@@ -26,7 +26,7 @@ package P to read the files of P's dependencies, only the compiled output
 of P.
 
 Usage:
-	6g [flags] file...
+	go tool 6g [flags] file...
 The specified files must be Go source files and all part of the same package.
 Substitute 6g with 8g or 5g where appropriate.
 

src/cmd/ld/doc.go

@@ -9,45 +9,52 @@ Ld is the portable code for a modified version of the Plan 9 linker.  The origin
 	http://plan9.bell-labs.com/magic/man2html/1/2l
 
 It reads object files (.5, .6, or .8 files) and writes a binary named for the
-architecture (5.out, 6.out, 8.out) by default.
+architecture (5.out, 6.out, 8.out) by default (if $GOOS is windows, a .exe suffix
+will be appended).
 
 Major changes include:
-	- support for ELF and Mach-O binary files
+	- support for ELF, Mach-O and PE binary files
 	- support for segmented stacks (this feature is implemented here, not in the compilers).
 
 Original options are listed on the manual page linked above.
 
+Usage:
+	go tool 6l [flags] mainObj
+Substitute 6l with 8l or 5l as appropriate.
+
 Options new in this version:
 
--d
+	-d
 		Elide the dynamic linking header.  With this option, the binary
-	is statically linked and does not refer to dynld.  Without this option
-	(the default), the binary's contents are identical but it is loaded with dynld.
--Hdarwin
+		is statically linked and does not refer to a dynamic linker.  Without this option
+		(the default), the binary's contents are identical but it is loaded with a dynamic
+		linker. This flag cannot be used when $GOOS is windows.
+	-Hdarwin     (only in 6l/8l)
 		Write Apple Mach-O binaries (default when $GOOS is darwin)
--Hlinux
+	-Hlinux
 		Write Linux ELF binaries (default when $GOOS is linux)
--Hfreebsd
+	-Hfreebsd    (only in 6l/8l)
 		Write FreeBSD ELF binaries (default when $GOOS is freebsd)
--Hnetbsd
+	-Hnetbsd     (only in 6l/8l)
 		Write NetBSD ELF binaries (default when $GOOS is netbsd)
--Hopenbsd
+	-Hopenbsd    (only in 6l/8l)
 		Write OpenBSD ELF binaries (default when $GOOS is openbsd)
--Hwindows
-	Write Windows PE32+ binaries (default when $GOOS is windows)
--I interpreter
+	-Hwindows    (only in 6l/8l)
+		Write Windows PE32+ Console binaries (default when $GOOS is windows)
+	-Hwindowsgui (only in 6l/8l)
+		Write Windows PE32+ GUI binaries
+	-I interpreter
 		Set the ELF dynamic linker to use.
--L dir1 -L dir2
+	-L dir1 -L dir2
 		Search for libraries (package files) in dir1, dir2, etc.
-	The default is the single location $GOROOT/pkg/$GOOS_amd64.
--r dir1:dir2:...
+		The default is the single location $GOROOT/pkg/$GOOS_$GOARCH.
+	-r dir1:dir2:...
 		Set the dynamic linker search path when using ELF.
--V
+	-V
 		Print the linker version.
--X symbol value
+	-X symbol value
 		Set the value of an otherwise uninitialized string variable.
 		The symbol name should be of the form importpath.name,
 		as displayed in the symbol table printed by "go tool nm".
-
 */
 package documentation

src/cmd/nm/doc.go

@@ -14,7 +14,8 @@ Plan 9 C compiler.
 This implementation adds the flag -S, which prints each symbol's size 
 in decimal after its address.
 
-It is installed as go tool nm and is architecture-independent.
+Usage:
+	go tool nm [-aghnsTu] file
 
 */
 package documentation